Profile measurement for underground hydrocarbon storage caverns

ABSTRACT

Underground storage caverns are widely used for the bulk storage of petroleum products, in particular, crude oil. The caverns are accessed through a casing in a borehole down to the cavern. The lower end of the casing opens into an upper region of the cavern termed the chimney. The chimney provides a transition from the casing into the cavern body. The invention presents a process of injecting a gas into the well while measuring the gas pressure and optionally measuring the volume of injected gas. The gas drives down an interface between the gas and hydrocarbon liquid. By monitoring the rate of change of the gas pressure, and detecting a sudden decrease in the rate of change, it can be determined when the interface has been driven down to the region immediately below the bottom of the casing at the upper end of the chimney.

RELATED U. S. APPLICATION DATA

This application is a divisional application of application Ser. No.15/621,574 filed on Jun. 13, 2017.

CROSS REFERENCE TO RELATED APPLICATION(S)

Applicant has filed copending applications entitled “Method forDetecting Leakage in an Underground Hydrocarbon Storage Cavern”, filedApr. 25, 2015 and having Ser. No. 14/696,387, Method for Determining theprofile of an Underground Hydrocarbon Storage Cavern, filed Apr. 25,2015 and having Ser. No. 14/696,389 (now U.S. Pat. No. 9,669,997) andMethod for Determining the profile of an Underground Hydrocarbon StorageCavern, filed May 2, 2017 and having Ser. No. 15/584,962 (now U.S. Pat.No. 10,323,971).

BACKGROUND 1. Field of the Invention

The field of the present invention is that of test and measurementequipment used in the oil and gas industry, which includes the use oflarge volume underground storage caverns for storing substantialquantities of petroleum products, such as crude oil, propane and refinedpetroleum products, and in particular to the determination of theconfiguration of such caverns.

2. Description of the Related Art

In the use of underground storage caverns, it is important to determinethe approximate shape and volume of the cavern or sections of thecavern. This has heretofore been done by lowering a wireline device intothe cavern and using sonic devices to measure distances from the deviceto the cavern wall. Another technique has been to pump a liquid into theannulus and determine cavern volume by measuring the liquid pressure andvolume at the annulus and central tubing at the well surface. Wirelineoperations are complex, expensive and subject to leakage of gas orliquid from the wellhead or wireline connectors. Prior cavern surveytechniques are shown in U.S. Pat. No. 2,792,708, issued May 21, 1957entitled “Testing Underground Storage Cavities” and U.S. Pat. No.3,049,920, issued Aug. 21, 1962 entitled “Method of Determining Amountof Fluid in Underground Storage”.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, reference is now made to the following descriptiontaken in conjunction with the accompanying original drawings in which:

FIG. 1 is an elevation profile view of an underground storage cavernwith an upper chimney section and installed casing and tubing forfilling and removing liquids that are stored in the cavern, along withequipment for injecting a gas, such as nitrogen, into the casing withsurface equipment for measuring the pressure and volume (mass) ofinjected gas,

FIG. 2 is a chart illustrating measurements of surface gas pressure as afunction of time,

FIG. 3 is a chart derived from the data in the chart shown in FIG. 2showing the rate of change in gas pressure as a function of time,

FIG. 4 is a chart illustrating measurements of casing gas pressure atthe earth surface as a function of the volume of injected gas, togetherwith two calculated rate of change (pressure/volume) measurements, Rate1 and Rate 2,

FIG. 5 is a chart derived from the chart in FIG. 4 illustrating therates of change of gas pressure as a function of volume of injected gas,and

FIG. 6 is a bar graph chart illustrating the calculated radius of thecasing and the upper chimney region of the cavern as a function of depthbelow the surface, which is an illustration of a chimney profile.

DETAILED DESCRIPTION OF THE INVENTION

Multiple embodiments of the present invention are now described inreference to the FIGS. 1-6. This invention is for use with anunderground storage cavern, which is also referred to as a storage well.

An important objective of the present invention is to determine when agas/liquid interface, which is driven downward by injection of gas intothe casing, is located at a position in a region immediately below thecasing shoe. The casing shoe is positioned at the bottom of the stringof casing in the storage well. The interface level is termed a referencelevel. This operation is a part of a process for measuring the profileof a chimney of an underground storage cavern.

Measuring the profile of a chimney of a storage cavern is importantbecause it can indicate the mechanical integrity of the chimney portionof the storage cavern. If the profile is measured periodically, forexample, every five years, each measurement can be compared to the lastmeasurement. If the present measurement is substantially the same, thechimney is likely to be maintaining structural integrity. But if thereis a substantial change, it is likely that the chimney has been damagedby a wall collapse, erosion, leakage or possibly blockage. The walls ofthe chimney are salt, which can dissolve, erode or break. A change inthe chimney can damage the casing, the casing shoe or weaken theformation above the chimney and lead to leakage of liquid or gas out ofthe cavern into the underground formation regions near the well. This inturn could lead to gas or liquid leakage at the earth surface, whichcould result in a fire or release of toxic gas into the atmosphere, orlead to ground water contamination.

Referring to FIG. 1, there is shown an underground storage cavern 10which has a section termed a chimney 12 at the upper end thereof and hasa cavern body 14 that is below the chimney and serves as the primarystorage region for the stored liquid. The chimney can be several hundredfeet high and the cavern can be over a thousand feet in depth andhundreds of feet wide. Such a cavern can have a capacity to hold severalmillion barrels of hydrocarbon liquid, for example, crude oil.

A casing 16 is installed to extend from a wellhead tree 15 at the earthsurface 18 down to the top of the chimney 12. A layer of caprock 19 liesbelow the earth surface 18. Below the caprock 19 and surrounding thecavern 10 is a salt formation 23. The cavern 10 is formed within thesalt formation 23.

The wellhead tree 15 of the well is located at the earth surface 18. Astructure termed a casing shoe 20 is positioned at the bottom of thecasing 16. The casing shoe 20 provides a transition from the lower endof the casing 16 into the chimney 12. A casing liner 21, made of cement,is formed on the outside of the casing 16 and the interior of the wellborehole. The depth of the casing shoe 20 in a particular well can befound in a log for that particular well and/or in a completion reportfor the well that is filed with the relevant authority. The cementcasing liner 21 serves as a barrier to the leakage of fluids (liquid orgas) from the interior of the chimney 12 into the earth formationsurrounding the casing 16. A string of tubing 22 is optionallypositioned inside the casing 16. The present invention is applicable toa storage well that includes the tubing 22 and a storage well that doesnot have a string of tubing installed inside the casing. The tubingextends from the wellhead tree 15 down to near the bottom of the cavernbody 14. The casing 16, tubing 22 and liner 21 extend through the layerof caprock 19. A liquid such as brine 24 is pumped into the cavern 10and settles below a liquid 26 because the brine 24 is more dense thanthe liquid 26. The liquid 26 can be a hydrocarbon liquid such as crudeoil. When brine is pumped down the tubing 22 from the surface, it servesto lift the liquid 26 upward through an annulus 28, which is a regionbetween the casing 16 and tubing 22, and ultimately to exit the well atthe surface through a flow line 32. There may be a gas/liquid interface30 between the liquid 26 and a gas 52 in the annulus 28 and thisinterface can extend down into the chimney 12 and cavern body 14. Aliquid/liquid interface 31 is located between the liquid 26 and the moredense brine 24.

The storage cavern 10 may have multiple casings positioned concentricabout the tubing 22. Typically, the outer casings extend less deep intothe earth formation than the innermost casing, such as casing 16.

Further referring to FIG. 1, a gas tank 34, preferably containingcompressed or liquified nitrogen, is coupled through a valve 36 to amass flow meter 38. An example of a mass meter 38 is a Micro MotionELITE Coriolis Flow Meter. The valve 36 can be set to have a constantflow rate of nitrogen gas from the tank 34 to the mass meter 38 and intothe casing 16. Another method for determining the mass of gas injectedinto the well is to weigh the gas tank 34 continuously or periodically.A weight measurement as a function of time indicates the flow rate ofgas into the well. A still further technique is to measure the pressureof the gas in tank 34 and calculate the volume of gas in the tank fromthis pressure measurement.

A pressure meter 40 is mounted to the casing 16 for measuring thepressure of the gas in the casing at the earth surface. The mass meter38 is connected through a data line 42 to a multichannel dataacquisition recorder 44 so that the mass readings can be recorded as afunction of time. Likewise, the pressure meter 40 is connected through adata line 46 to the recorder 44 for recording pressure measurements.Wireless links can be used in place of the data lines if desired. Thesurface gas pressure and gas mass readings are correlated with eachother as shown in FIG. 2.

The meter 38 directly measures the mass of gas that passes through themeter. The mass reading can be converted to volume by using thewell-known gas law equations. The gas volume (mass) measurement isexpressed in SCF (Standard Cubic Foot).

The temperature of the gas in the casing 16 at the earth surface ismeasured by a thermometer 48 and the measured temperature readings aresent through a data line 50 to the recorder 44.

The recorder 44 is coupled to a computer 49 through a data line 51 toprovide the data collected from the meter 38, meter 40 and thermometer48 to the computer 49 for processing and display, as further describedbelow.

Further referring to FIG. 1, the gas injected into the casing-tubingannulus 28 is indicated by the reference numeral 52. The interface 30 isshown in the annulus 28 and can be located at any depth in the casing16. The interface 30 can initially be at the top of the casing 16 at theearth surface 18 and then driven downward into the well as gas 52 isinjected into the casing 16 from the tank 34. Representative depthlocations of the interface 30 are shown by the reference numerals 62,64, 66, and 68. The interface 30 can be driven down into the chimney 12and the cavern body 14.

A first embodiment of the invention is now described in reference toFIGS. 1, 2 and 3. In this embodiment the valve 36 is set to inject aconstant rate of flow of nitrogen gas from tank 34 into the casing 16.For the illustrated example, this rate is 1,000 SCF/min. The flow of gas52 causes the gas pressure in the casing 16 at the earth surface toincrease. This is shown in FIG. 2 which is a chart illustrating theincrease in casing gas pressure, the vertical scale, as a function oftime, the horizontal scale. A data line 70 illustrates the measuredvalues of gas pressure as a function of time. A casing pressure rate ofchange is calculated by subtracting a gas pressure at a first time froma gas pressure at a second time and dividing this pressure difference bythe interval of time between the first time and the second time. Thischart shows that starting at time minute 51, the casing pressureincreases at a constant rate until it reaches time 53:04. This rate isapproximately 11 psi/min. After time 53:04 the rate of change is at alesser rate which decreases with time. This is shown by line 72.

The values for the data shown in FIG. 2, together with the rate ofchange of pressure is shown in Table 1 below. The rate of change iscalculated for each 12 second interval, and is expressed in psi/min. Thepressure values (P1, P2, P3, . . . ) are taken at respective times (T1,T2, T3, . . . ). The calculation for pressure rate of change per unit oftime (ΔP1, ΔP2, ΔP3 . . . ) is (ΔP1=[P2-P1]/[T2-T1],ΔP2=[P3-P2]/[T3-T2], . . . ), as shown in the following Table 1.

TABLE 1 Time T Pressure P Rate of Pressure Change (min:sec) (psi) (ΔPpsi/min) 51:00  1364.5 — :12 1366.7 11 :24 1368.9 11 :36 1371.1 11 :481373.3 11 52:00  1375.5 11 :12 1377.7 11 :24 1379.9 11 :36 1382.1 11 :481384.3 11 53:00  1386.5 11 :12 1387.3 4 :24 1387.8 2.5 :36 1388.0 1.0

The rate of change values for the data shown in FIG. 2 are illustratedin the chart shown in FIG. 3. The calculated rate of change values areplotted as a function of time. The rate of change value is essentiallyconstant from time 51 to time 53 at a value of 11 psi/min and then itrapidly drops to 4 psi/min at 53:12, then 2.5 psi/min at 53:24 then downto 1.0 psi/min at 53:36.

Referring back to FIG. 1 illustrates the cause for the change in the gaspressure rate of change while a constant flow rate of gas 52 is injectedinto the casing 16 at the surface from time 51 minutes until time 54minutes. It has been found that in caverns such as 10, that a change inpressure of the gas in a well at a given depth causes a change in thedepth of the interface that is a function of the pressure change and thegradient of the liquid at that depth. At time 51 the interface 30 islocated at, or near, the earth surface 18. As the gas 52 is initiallyinjected into the casing 16, the interface 30 is located in the annulus28. This annulus, as shown in FIG. 1, has a constant cross section sizefrom the earth surface 18 down to the casing shoe 20 at the bottom endof the casing 16. However, if the casing 16 has been damaged, a casingliner may be installed and this liner will have a lesser diameter.Within the reduced diameter section the rate of change of gas pressurewill be constant, but the value will be different. Within any section ofcasing with a constant diameter, the rate of change of gas pressure willbe constant. A given volume of gas injected into the casing repeatedlycauses the same change in pressure because the geometry within theannulus 28 is constant. A given volume of injected gas depresses theinterface 30 the same distance each time that the given volume of gas isinjected. The rate of change of pressure is constant as long as theinterface 30 is in the annulus 28. In FIG. 2, the interface 30 reachesthe bottom of the casing 16 at the 53:04 time mark (indicated by arrow88) and then enters into the top of the chimney 12. The top of thechimney 12 has a significantly larger cross section area in comparisonto that of the annulus 28 and therefore a larger volume per unit ofdepth. Due to the larger volume of the chimney 12 the given volume ofinjected gas depresses the interface 30 a shorter distance as shown andtherefore there is a smaller change in pressure. As shown in FIG. 3, therate of pressure change from 51 min. to 53 min is 11 psi/min, but afterthe 53:04 time mark, the rate of pressure change per unit of time dropsto 4 psi/min, then 2.5 psi/min and then to 1.0 psi/min. This correspondsto the interface 30 leaving the constant cross section annulus 28 andentering into the top region of the chimney 12 that exhibits a largercross-sectional area.

A first technique for determining when the interface 30 leaves thebottom of the casing 16 and enters into the top of the chimney 12 is tocompare each calculated pressure rate of change value to the immediatelypreceding pressure rate of change value and determine when a pressurerate of change value is initially less than a predetermined percentageof the value of the preceding rate of change value. If the predeterminepercentage change is selected to be 50%, each pressure rate of changevalue is compared to the preceding rate of change value. For each of thevalues shown in Table 1 from time 51 to time 53, the percentage changefor each value from the previous value is 0%. But from time 53:00 min totime 53:12 min, the pressure rate of change goes from 11 to 4. This is areduction of 64%. With a threshold set at 50%, this indicates that theinterface 30 entered into the top region of the chimney 12 during thetime from 53:00 min to 53:12 min.

This example uses 12 seconds as the interval for calculating pressurerate of change, however, other intervals, longer or shorter, can also beused.

Another technique for determining when the interface 30 leaves thecasing 16 and enters into the top region of chimney 12 is to compareeach pressure rate of change value to an earlier pressure rate of changevalue that is not the immediately preceding value. For example, eachvalue could be compared to the second preceding value. In the aboveexample, there would be the same result because the second precedingvalue is 11 for comparison to the present value of 4. This techniquecould be preferred if the change in area from the annulus 28 at the endof the casing 16 into the top region of the chimney 12 is more gradualand therefore the amount of the rate of pressure change is less fromsample to sample. See Table 2 below.

TABLE 2 Rate of Pressure Change Time (ΔP psi/min) 51:00  — :12 11 :36 11:48 11 52:00  11 :12 10 :24 9 :36 6 :48 6 53:00  4 :12 3 :24 3

Referring to the data in Table 2, for a rule that sets the comparison ofeach rate of pressure change value to the third preceding value with atleast a 50% reduction, the value “4” is the value in the time sequencethat meets this rule. This rule indicates that the interface 30 enteredinto the upper region of the chimney 12 during the time interval from52:24 to 53:00.

A still further technique is to compare the present value to a runningaverage of prior values. For example, the present value could becompared to the average of the preceding four rate of change values. SeeTable 2 above. With a 30% threshold, the first value that is less than30% of the running average of the four preceding four values is “6”. Theaverage of the four preceding values is 10 and 6 is 40% less than 10.Using this rule, the interface 30 is indicated to have entered into theupper region of the chimney 12 during the time interval between 52:24and 52:36.

The rule to use, and the percentage change to use, in a particularapplication can depend on the known or anticipated geometry of the wellor the nature of the data that has been collected.

One rule is to use the average of multiple values and compare to apresent measurement of rate of gas pressure change. A change from theaverage of 30% or 50% can indicate the inflection point. This detectedchange will be close to the actual point where the interface enters intothe chimney. A running average of seven preceding values in Table 2 withat least a 40% difference less than the average selects the value “6” at52:48.

A further embodiment of the present invention is now described inreference to FIGS. 4 and 5. For detecting the interface 30 position atthe reference level, which is in the top region of the chimney 12, thisembodiment utilizes the rate of change in gas pressure as a function ofthe cumulative volume of gas pumped into the casing 16, in contrast tothe embodiment described in reference to FIGS. 2 and 3 which is based ona rate of change of gas pressure as a function of time. Referring toFIG. 4, a data line 80 is a plot of the standard volume (mass) ofinjected gas along the horizontal axis and the gas pressure in thecasing 16 at the earth surface 18 along the vertical axis. For this setof data, the line 80 has essentially a constant average rate of change(Rate 1) of 10.8 psi/100 SCF from volume 0 to volume 210 SCF at line 82.After volume 210 SCF, the average rate of change goes down toapproximately 1.25 psi/100 SCF (Rate 2).

For this embodiment, the rate of flow of gas 52 injected into the casing16 need not be a constant rate, it can vary with time. Data pointstogether with calculated gas pressure rates of change as a function ofcumulative gas volume are shown in Table 3 below. The gas pressure rateof change (ΔP1, ΔP2, . . . ) is determined by measuring a series of gasvolume measurements (V1, V2, V3 . . . ) and simultaneous timecorresponding gas pressure measurements (P1, P2, P3 . . . ). The rate ofgas pressure change is calculated as (ΔP1=[P2-P1]/[V2-V1],ΔP2=[P3-P2]/[V3-V2] . . . ).

TABLE 3 Volume Injected V Gas Pressure P Rate of Gas Pressure Change ΔP(SCF) (Psig) (psi/100 SCF) 0 1364.0 — 25 1366.7 10.8 50 1369.4 10.8 751372.1 10.8 100 1374.8 10.8 125 1377.5 10.8 150 13180.2 10.8 175 1382.910.8 200 1385.6 10.8 225 1387.4 1.8 250 1389.0 1.6 275 1390.25 1.25 3001391.5 1.25

For this embodiment, the methods for detecting when the interface 30 hasentered into the top region of the chimney 12 are the same as describedabove. First technique is when a gas pressure rate of change value isless than a predetermined percentage of an immediately preceding value.If the predetermined percentage is 50%, the identified rate of changevalue in Table 3 is 1.8 which corresponds to the injected gas volume of225 SCF. If the value is compared to a third preceding value, the resultis also the 1.8 value. The comparison of a running average of thepreceding four rate of change of gas pressure values with apredetermined percentage of 30% also selects the 1.8 psi/100SCF value.This selection indicates that the interface 30 enters into the topmostregion of the chimney 12 between the measurements of 200 SCF and 225SCF. A running average of seven prior values with at least a 40% changedeems the 1.8 value as the transition reading.

FIG. 5 is a chart with data curve 86 showing the relation of thecumulative volume of injected gas 52 with the rate of change of gaspressure in the casing 16 at the earth surface 18. The gas injection isstarted when the interface 30 is located at or near the earth surface.As the interface 30 is pushed downward in the casing annulus 28, therate of increase in surface pressure is uniform because the geometry ofthe annulus cross section is constant, if the internal diameter of thecasing remains constant. When approximately 200 SCF of gas has beeninjected, the pressure rate of change begins to drop substantially as afunction of the volume of injected gas. Note that the gas pressure doesnot drop, it is the rate of change in the gas pressure that drops. Thisis due to the interface 30 entering into the top region of the chimney12 which has a much larger volume per unit of depth than that of thecasing annulus 28. Much more gas is required to lower the interface agiven distance than was needed to lower the interface such a givendistance in the casing annulus 28.

Multiple embodiments of the invention are described above to detect whenthe interface 30, which is driven downward into the well by theinjection of gas, passes through the bottom end of the casing 16 intothe top region of the chimney 12, by identifying when a sudden changeoccurs in the gas pressure rate of change, in comparison to either timeor volume of injected gas. When the interface 30 is located immediatelybelow the casing shoe 20, typically within two to five feet, this istermed the reference level of the interface 30. After the interface 30has been determined to be at this reference level, further steps inaccordance with the invention are to measure the volumes of sections ofthe chimney 12 located below the reference level. This constitutesestablishing a profile of the chimney 12.

Referring to FIG. 1, the interface 30 can be driven downward to thedepth 62, which is the reference level detected as described above. Thenext measurements and calculations are directed to determining thevolume of the chimney 12 at measured depths. In particular, themeasurements are directed to determining change in depth values (Δd1,Δd2, Δd3 . . . ) based on changes in gas pressure measurements at thesedepths in the cavern storage well. The pressure measurements at thesedepths are termed Pd1, Pd2, Pd3, . . . . The change in gas pressuremeasurements at these depths are termed ΔPd1, ΔPd2, ΔPd3 . . . . Thechanges in depths are calculated by the formula Δd1=ΔPd1/G, Δd2=ΔPd2/G,. . . . G is the gradient (psi/foot) of the hydrocarbon liquid. Thegradient G can vary with depth. The gradient (psi/foot) of the liquid 26is either measured for the well under test or determined by reference tostandard values for the type of liquid 26 in the well.

The pressure at a particular depth is based on the surface pressuremeasurement of the gas pressure in the casing 16 at the earth surface.The pressure at a depth, such as depth 62, shown in FIG. 1. is thesurface measured pressure plus the pressure due to the weight of the gascolumn from the surface down to the depth 62. The weight of this columnis determined by the length of the column, the chemical composition ofthe gas, the pressure of the gas and the temperature of the gas. Thesecalculations for down-hole pressure are well known in the art and widelyused in the oil and gas industry. Table 4 below illustrates downholepressure calculated from surface pressure for a particular well. Similarcalculations can be made for any storage cavern well.

TABLE 4 Depth Interface Pressure Surface Pressure P (feet) at DepthPd(Psig) (Psig) 0 773.7 773.7 100 809.8 807.2 197 844.8 839.6 300 881.9873.8 400 918.0 907.0 500 930.1 940.0 600 990.1 972.8 748 1043.5 1020.9800 1062.3 1037.7 900 1098.3 1069.3 1000 1134.4 1100.7 1100 1170.51131.7 1200 1206.5 1162.3 1300 1242.6 1192.6 1400 1278.7 1223.2 14501296.7 1238.4 1500 1314.8 1253.6 1600 1350.8 1283.7 1700 1386.7 1313.61800 1423.0 1343.3 1900 1459.0 1372.7 1970 1484.3 1393.1 1985 1489.71397.5 2000 1495.1 1401.8

The actual volume of gas at a depth in the well due to the injection ofgas at the surface is less than the measured standard volume of gasinjected at the surface due to the greater gas pressure and temperatureat the depth. The calculation of the volume of gas at depths in the wellis well known in the art and widely used in the oil and gas industry.The standard volume (mass) of injected gas injected at the surfacebetween two points in time is known together with the surface pressureand temperature. The pressure and temperature at depth are known. Thetemperature at each depth is available from a temperature surveypreviously taken for the well, or known for a geographic region. Theactual volume at depth is calculated by use of the gas law equationsusing all of these parameters, which are the standard volume, at thesurface pressure and temperature, and the at depth pressure andtemperature. See Table 5 below showing the standard volume of gasinjected at the surface and the corresponding actual volume at givendepths. For example, for 49.8 SCF of N2 injected at the surface, thereis a one cubic foot displacement at 0 depth (the earth surface). Butwhen the interface is at, for example 1400 feet, there must be aninjection at the surface of 74.0 SCF of N2 for a of one cubic footvolume of gas at 1400 feet. The at-depth volume of gas is based on thestandard volume (mass) of gas injected at the surface.

TABLE 5 Depth SCF of N2 Gas/Cu. Ft (feet) (Avg. P & T)) 0 49.8 100 51.4197 52.9 300 30.5 400 56.0 500 57.4 600 58.9 748 61.5 800 62.2 900 64.01000 65.8 1100 67.8 1200 69.8 1300 71.9 1400 74.0 1450 75.1 1500 76.11600 78.2 1700 80.2 1800 82.3 1900 84.3 1970 85.6 1985 86.1 2000 86.5

Referring to FIG. 1, when the interface 30 has been depressed to depth62, the reference level, the surface gas pressure is measured andadditional gas is injected. A second gas pressure measurement is madeand the standard volume of gas injected between the gas pressuremeasurements is determined. The two surface pressure measurements areused to determine the at-depth pressure values, as discussed above. Thedifference in these two pressure values (Pd2−Pd1) is multiplied by thegradient G of the hydrocarbon liquid and the product is the distance(Δd1) that the interface moved between the two pressure measurements.Δd1 is the distance that the interface 30 moved downward from thereference level. If the Δd1 value is 10 feet, the resulting depth of theinterface is depth 64 as shown in FIG. 1. The incremental standardvolume of gas injected at the surface is used to determine the actualvolume of liquid displaced by the additional gas ΔV1 between the depths62 and 64. The measured profile volume of the chimney 12 between depths62 and 64 is a height of Δd1 and a volume of ΔV1. Assuming that thechimney 12 has a cylindrical geometry and there is no tubing string inthe cavern, the cavern radius (rc) is determined by the formula(rc=√(V÷(Δd π)) This radius is illustrated in FIG. 6 for each depthinterval. If there is a tubing string present in the cavern, and thetubing string has a radius of rt, the cavern radius rc is (rc=√(V÷(Δdπ)+rt²)

When the interface 30 has been driven down to the depth 64 and thepressures and gas volume has been recorded, more gas is injected todrive the interface 30 further downward. A new at-depth pressure isdetermined from a surface measurement and the volume of gas injected atthe surface is measured and used to determine the volume of gas at thedepth between the last two pressure measurements. The depth of movementΔd2 is determined as described above using the pressure differential atthe depths and the gradient of the liquid 26. This determines the heightand volume for another profile section of the chimney 12. The radius forthis profile section of chimney 12 is then calculated.

FIG. 6 is a bar graph that shows the radius of the chimney 12(horizontal axis) as a function of the depth (vertical axis) in thecavern storage well. Each of the bars 92, 94, 96, 98 and 100 represent aradius in the casing 16 or the chimney 12 at the indicated depths. Forexample, bars 92 and 94 represent the radius of the casing 16. Thiscalculation takes into consideration the cross-section area of thetubing 22 and therefore the volume of the tubing 22. Bars 98 and 100indicate an average radius in the chimney 12 of approximately 50 inchesat depth range 1985-1995 for bar 98 and depth range 1995-2005 for bar100. The radius values are produced as described above. The bar graph inFIG. 6 can be used as a reference to compare to future profilemeasurements for the chimney 12 to evaluate the mechanical integrity ofthe chimney 12 over time.

Although several embodiments of the invention have been illustrated inthe accompanying drawings and described in the foregoing DetailedDescription, it will be understood that the invention is not limited tothe embodiments disclosed, but is capable of numerous rearrangements,modifications and substitutions without departing from the scope of theinvention.

1. A method for use in a cavern storage well which has a casing thatextends from the earth surface down to a chimney region that extendsdownward and opens into a cavern body wherein hydrocarbon liquid isstored in the cavern body above a liquid more dense than the hydrocarbonliquid, the method indicating when an interface between a gas and thehydrocarbon liquid is located at the top of said chimney a shortdistance below the lower end of said casing, comprising the steps of:injecting the gas into said casing at the earth surface at a constantrate of flow to drive the interface downward, measuring the pressure ofsaid gas in said casing at the earth surface to produce a series of gaspressure measurements (P1, P2, P3 . . . ) at a sequence of respectivetimes (T1, T2, T3 . . . ), producing a series of gas pressure rate ofchange values (ΔP1, ΔP2, . . . ) based on said gas pressure measurementsand time intervals between said times for adjacent pairs of said gaspressure measurements (ΔP1=[P2−P1]/[T2−T1], ΔP2=[P3−P2]/[T3-T2] . . . ),and comparing each of a group of said gas pressure rate of change values(ΔP1, ΔP2, . . . ) to a preceding one of said gas pressure rate ofchange values to detect when a gas pressure rate of change value isinitially less than a predetermined percentage of said preceding one ofsaid gas pressure rate of change values, thereby indicating that saidinterface is located within the top region of said chimney below thelower end of said casing between the times when said less than apredetermined percentage gas pressure rate of change value gas pressuremeasurements were made.
 2. The method recited in claim 1 wherein saidpreceding one of said gas pressure rate of change value is theimmediately preceding gas pressure rate of change value before said gaspressure rate of change value which was detected to have a value that isinitially less than a predetermined percentage of a preceding gaspressure rate of change value.
 3. The method recited in claim 1 whereinsaid preceding one of said gas pressure rate of change value is one ofthe preceding gas pressure rate of change values other than theimmediately preceding gas pressure rate of change value before saidpressure rate of change value which was detected to have a value that isinitially less than a predetermined percentage of a preceding gaspressure rate of change value.
 4. The method recited in claim 1including the following steps for producing a profile of said chimney:after said interface has been located at the top region of said chimneybelow the lower end of said casing, further injecting said gas into saidcasing at said earth surface and measuring said gas pressure at theearth surface to produce a series of depth gas pressure measurements(Pd1, Pd2, Pd3, . . . ), measuring the mass of said gas injected intosaid casing between each pair of depth gas pressure measurements,determining a series of change in depth values (Δd1, Δd2, Δd3 . . . ),each depth value based on a gas pressure change value (ΔPd1, ΔPd2, . . .) between two adjacent depth gas pressure measurements (ΔPd1=Pd2−Pd1,ΔPd2=Pd3−Pd2, . . . ) and a gradient (G pressure/distance) value of saidhydrocarbon liquid, wherein the change in depth values are (Δd1=ΔPd1/G,Δd2=ΔPd2/G, . . . ), and wherein a series of profile measurements ofsaid chimney are produced, each said profile measurement defined by (1)a change in depth value and (2) a volume of gas caused by said mass ofsaid gas injected into said casing between each pair of correspondingdepth gas pressure measurements which define the change in depth value(1).
 5. The method recited in claim 1 wherein said predeterminedpercentage is 50%.
 6. A method for use in a cavern storage well whichhas a casing that extends from the earth surface down to a chimneyregion that extends downward and opens into a cavern body whereinhydrocarbon liquid is stored in the cavern body above a liquid moredense than the hydrocarbon liquid, the method indicating when aninterface of a gas with the hydrocarbon liquid is located at the top ofsaid chimney a short distance below the lower end of said casing,comprising the steps of: injecting the gas into said casing at the earthsurface at a constant rate of flow to drive said interface downward,measuring the pressure of said gas in said casing at the earth surfaceto produce a series of gas pressure measurements (P1, P2, P3 . . . ) ata sequence of corresponding times (T1, T2, T3 . . . ), producing aseries of gas pressure rate of change values (ΔP1, ΔP2, . . . ) based onsaid gas pressure measurements and time intervals between said times foradjacent pairs of said gas pressure measurements (ΔP1=[P2−P1]/[T2−T1],ΔP2=[P3−P2]/[T3−T2] . . . ), and comparing each of said gas pressurerate of change values (ΔP1, ΔP2, . . . ) to a running average value of aplurality of preceding ones of said gas pressure rate of change valuesto detect when a gas pressure rate of change value is initially lessthan a predetermined percentage of the running average value, therebyindicating that said interface is located within the top region of saidchimney below the lower end of said casing between the times when saidgas pressure measurements were made for the less than predeterminedpercentage gas pressure rate of change value.
 7. The method recited inclaim 6 including the following steps for producing a profile of saidchimney: after said interface has been located at the top region of saidchimney below the lower end of said casing, further injecting said gasinto said casing at said earth surface and measuring said gas pressureat the earth surface to produce a series of depth gas pressuremeasurements (Pd1, Pd2, Pd3, . . . ), measuring the mass of said gasinjected into said casing between each pair of depth gas pressuremeasurements, determining a series of change in depth values (Δd1, Δd2,Δd3 . . . ), each depth value based on a gas pressure change value(ΔPd1, ΔPd2, . . . ) between two adjacent depth gas pressuremeasurements (ΔPd1=Pd2−Pd1, ΔPd2=Pd3−Pd2, . . . ) and a gradient (Gpressure/distance) value of said hydrocarbon liquid, wherein the changein depth values are (Δd1=ΔPd1/G, Δd2=ΔPd2/G, . . . ), and wherein aseries of profile measurements of said chimney are produced, each saidprofile measurement defined by (1) a change in depth value and (2) saidmass of said gas injected into said casing between each pair ofcorresponding depth gas pressure measurements which define the change indepth value (1).
 8. The method recited in claim 6 wherein said runningaverage comprises seven of said gas pressure rate of change values.
 9. Amethod for use in a cavern storage well which has a casing that extendsfrom the earth surface down to a chimney region that extends downwardand opens into a cavern body wherein hydrocarbon liquid is stored in thecavern body above a liquid more dense than the hydrocarbon liquid, themethod indicating when an interface of gas with the hydrocarbon liquidis located at the top of said chimney a short distance below the lowerend of said casing, comprising the steps of: injecting a gas into saidcasing at the earth surface to drive said interface downward, measuringthe volume of said gas injected into said casing to produce a series ofgas volume measurements (V1, V2, V3 . . . ) at respective times T1, T2,T3 . . . measuring the pressure of said gas in said casing at the earthsurface to produce a series of gas pressure measurements (P1, P2, P3 . .. ) at times (T1, T2, T3, . . . ) with said gas volume measurements,producing a series of gas pressure rate of change values (ΔP1, ΔP2, . .. ) based on said gas pressure measurements and said gas volumemeasurements wherein (ΔP1=[P2−P1]/[V2−V1], ΔP2=[P3−P2]/[V3−V2] . . . ),and comparing each of said gas pressure rate of change values (ΔP1, ΔP2,. . . ) to a preceding one of said pressure rate of change values todetect when a gas pressure rate of change value is initially less than apredetermined percentage of a preceding gas pressure rate of changevalue, thereby indicating that said interface is located within the topregion of said chimney below the lower end of said casing between thetimes when the gas pressure measurements were made for said gas pressurerate of change value that is initially less than a predeterminedpercentage of a preceding gas pressure rate of change value.
 10. Themethod recited in claim 9 wherein said gas volume measurements are basedon measuring the mass of said gas injected into said casing.
 11. Themethod recited in claim 9 including the following steps for producing aprofile of said chimney: after said interface has been located at thetop of region said chimney below the lower end of said casing, furtherinjecting said gas into said casing at said earth surface and measuringsaid gas pressure at the earth surface to produce a series of depth gaspressure measurements (Pd1, Pd2, Pd3, . . . ), measuring the mass ofsaid gas injected into said casing between each pair of depth gaspressure measurements, determining a series of change in depth values(Δd1, Δd2, Δd3 . . . ), each depth value based on a gas pressure changevalue (ΔPd1, ΔPd2, . . . ) between two adjacent depth gas pressuremeasurements (ΔPd1=Pd2−Pd1, ΔPd2=Pd3−Pd2, . . . ) and a gradient (Gpressure/distance) value of said hydrocarbon liquid, wherein the changein depth values are (Δd1=ΔPd1/G, Δd2=ΔPd2/G, . . . ), and wherein aseries of profile measurements of said chimney are produced, each saidprofile measurement defined by (1) a change in depth value and (2) avolume of gas caused by said mass of said gas injected into said casingbetween each pair of corresponding depth gas pressure measurements whichdefine the change in depth value (1).
 12. The method recited in claim 9wherein said predetermined percentage is 50%.
 13. A method for use in acavern storage well which has a casing that extends from an earthsurface down to a chimney region that has a top region which is adjacentto a lower end of said casing, the chimney region extends downward andopens into a cavern body wherein hydrocarbon liquid is stored in thecavern body above a liquid more dense than the hydrocarbon liquid, themethod indicating when an interface of a gas, which has mass, with thehydrocarbon liquid is located at the top region of said chimney region ashort distance below the lower end of said casing, comprising the stepsof: injecting the gas, by application of pressure to the gas, into saidcasing at the earth surface to drive said interface downward, measuringthe volume of said gas injected into said casing to produce a series ofgas volume measurements (V1, V2, V3 . . . ), measuring the pressure ofsaid gas in said casing at the earth surface to produce a series of gaspressure measurements (P1, P2, P3 . . . ) that correspond in time withsaid gas volume measurements, producing a series of gas pressure rate ofchange values (ΔP1, ΔP2, . . . ) based on said gas pressure measurementsand said gas volume measurements (ΔP1=[P2−P1]/[V2−V1],ΔP2=[P3−P2]/[V3−V2] . . . ), and comparing each of said gas pressurerate of change values (ΔP1, ΔP2, . . . ) to a running average value of aplurality of preceding ones of said gas pressure rate of change valuesto detect when one of said gas pressure rate of change values isinitially less than a predetermined percentage of the running averagevalue, thereby indicating that said interface is located within the topregion of said chimney region below the lower end of said casing whensaid gas pressure measurements were made for the less than apredetermined percentage gas pressure rate of change value.
 14. Themethod recited in claim 13 wherein said gas volume measurements arebased on measuring the mass of said gas injected into said casing. 15.The method recited in claim 13 including the following steps forproducing a profile of said chimney: after said interface has beenlocated at the top region of said chimney region below the lower end ofsaid casing, further injecting said gas into said casing at said earthsurface and measuring said gas pressure at the earth surface to producea series of depth gas pressure measurements (Pd1, Pd2, Pd3, . . . ),measuring the mass of said gas injected into said casing between eachpair of said depth gas pressure measurements, determining a series ofchange in depth values (Δd1, Δd2, Δd3 . . . ), each depth value based ona gas pressure change value (ΔPd1, ΔPd2, . . . ) between two adjacentdepth gas pressure measurements (ΔPd1=Pd2−Pd1, ΔPd2=Pd3−Pd2, . . . ) anda gradient (G pressure/distance) value of said hydrocarbon liquid,wherein the change in depth values are (Δd1=ΔPd1/G, Δd2=ΔPd2/G, . . . ),and wherein a series of profile measurements of said chimney areproduced, each said profile measurement defined by (1) a change in depthvalue and (2) a volume of gas caused by said mass of said gas injectedinto said casing between each pair of corresponding depth gas pressuremeasurements which define the change in depth value (1).
 16. The methodrecited in claim 13 wherein said predetermined percentage is 50%. 17.The method recited in claim 13 wherein the plurality of preceding onesof said gas pressure rate of change values has seven values.